This invention relates to the preparation of high-impact vinyl aromatic polymers, and more particularly to an improved two-step polymerization process for the preparation of vinyl aromatic polymer in bead form.
It is known that the impact resistance of vinyl aromatic polymers is improved by modification with a rubber, generally present in particulate form.
When a vinyl aromatic compound is polymerized in the presence of a rubber, the resulting compositions consist of said rubber and a vinyl aromatic polymer, a part of the latter being grafted on the rubber.
Procedures for producing the high-impact vinyl aromatic polymers require specific conditions for the pre-polymerization, copolymerization, or graft polymerization of the vinyl aromatic compound and rubber compound.
Known polymerization processes such as emulsion-, suspension-, or mass-polymerization may be used to prepare these polymeric compositions. A two-step process is often used. Such method involves first forming a mass polymer by pre-polymerizing a vinyl aromatic compound in the presence of a rubber and then suspending the mass to complete the grafting as a suspension. The polymeric composition is thus obtained in the form of beads which are easily separated from the reaction mixture and then washed and dried.
In the suspension polymerization step, the pre-polymerization mixture is suspended in an aqueous solution containing essentially a suspending agent and a wetting agent.
However, producing a stable aqueous suspension of the mass prepolymer is a critical operation. The stability of the suspension depends upon the amount and type of suspending agent, on the amount and type of wetting agent, on the size of the reactor, and on other factors. It has been observed more particularly that the amount of wetting agent must be kept between narrow limits and that these limits depend upon many factors. For instance, a suspension method which is suitable for one reactor cannot be applied in another larger reactor; in some cases, a suspension cannot be obtained and, in other cases, the polymeric composition forms beads which are too large and worked with difficulty.